1. Field of the Invention
The present invention relates to an ink set having an excellent image fastness, a container for storing the ink set, an inkjet recording method using the ink set, and a method for preventing discoloration of an inkjet-recorded image.
2. Description of the Related Art
In recent years, as an image-recording material there has been mainly used a material for forming color images. In particular, inkjet recording materials, heat-sensitive transfer image recording materials, recording materials using electrophotography, transfer silver halide photographic materials, printing inks, recording pens, etc. have been widely used.
These color image recording materials comprise dyes (dyestuff and pigment) of so-called subtractive primaries to reproduce or record a full-color image. However, under the present circumstances that there are no dyes which have absorption characteristics that can realize desired color reproduction region and are fast enough to withstand various working conditions. It has been keenly desired to improve these color image recording materials.
Due to low material cost, the possibility of high speed recording, low noise levels during recording and ease of color recording, the inkjet recording method has rapidly been diffused and further developed.
Inkjet recording methods are divided into two groups, i.e., continuance processes involving continuous flying of droplets and on-demand processes involving flying of droplets according to image data signals. Ejection processes for these inkjet recording methods are divided into four groups, i.e., processes which comprises applying pressure from a piezoelectric element to eject droplets, processes which comprises thermally generating air bubbles in the ink to eject droplets, processes using ultrasonic waves, and processes involving electrostatic suction and ejection of droplets. As an ink for inkjet recording an aqueous ink, oil-based ink or solid (melt type) ink may be used.
The coloring agents to be incorporated in such an ink for inkjet recording are required to have a good solubility or dispersibility in solvents, allow high density recording, exhibit a good hue, have a high fastness with respect to light, heat, active gases in the atmosphere (oxidizing gas such as NOx and ozone, SOx), water and chemicals, have a good fixability to the image material and hence be unlikely to ooze, have a high preservability in ink form, have no toxicity, have a high purity and a have good availability. However, it is extremely difficult to find coloring agents which can satisfy these requirements to a great extent. In particular, these coloring agents are keenly required to have a good hue of the three primary colors and a good fastness with respect to light, humidity, heat, and especially with respect to oxidizing gases such as ozone in the atmosphere, upon printing on an image-receiving material having an ink-receiving layer containing a porous particulate white inorganic pigment.
As magenta dyes, azo dyes comprising phenol, naphthol, aniline or the like as a coupling component have heretofore been widely used. As azo dyes having a good hue dyes disclosed in Japanese Patent Application Laid-open (JP-A) No. 11-209673, Japanese Patent No. 3,020,660, etc. are known. However, these dyes are disadvantageous in that they have a deteriorated light fastness. As improvements in these dyes, dyes having a good hue and an improved light fastness have been recently disclosed in JP-A No. 2001-335714. However, the dyes known in these patents leave much to be desired in fastness with respect to oxidizing gases such as ozone.
Representative examples of cyan dyes include phthalocyanine dyes and triphenylmethane dyes.
Representative examples of phthalocyanine-based dyes which have been most widely used include C. I. Direct Blue 86, 87, and 199. These phthalocyanine-based dyes are superior to magenta and yellow dyes in light fastness but exhibit a remarkable decoloration or discoloration due to oxidizing gases such as nitrogen oxide gas and ozone, which have recently been often raised as environmental issues.
To date, phthalocyanine-based dyes which are rendered fast with respect to ozone gas have been disclosed in JP-A No. 3-103484, JP-A No. 4-39365, JP-A No.2000-303009, etc. However, all these phthalocyanine-based dyes leave much to be desired in improvement of fastness with respect to oxidizing gas and thus further improvements thereto are desired.
On the other hand, triphenylmethane-based dyes such as Acid Blue 9 exhibit a good hue but have a drastically deteriorated fastness with respect to light and ozone gas.
As yellow dyes, azobenzene-based dyes such as Direct Yellow 86 and 120, pyrazolonazo dyes such as Acid Yellow 17, and heterocyclic azo dyes such as pyridonazo dyes have been used. Further, quinophthalone-based dyes have been often proposed. However, these dyes which have heretofore been known are disadvantageous in that most of those having a good hue, and particularly a sharp shoulder of absorption characteristic curve on the long wavelength side, such as quinophthalone dyes are not fast with respect to ozone and light. Azobenzene-based dyes have a good fastness but a sloping shoulder of absorption characteristic curve on the long wavelength side. Thus, there are no yellow dyes which can satisfy both the hue and fastness requirements.
Representative examples of black dyes include Food Black 2, bisazo dye, and trisazo dye. However, there are no black dyes which are excellent both in light fastness and ozone fastness. On the other hand, pigments such as carbon black are widely used. JP-A No.3-210373 discloses an ink comprising an acidic carbon black and an alkali-soluble polymer. Further, JP-A No.3-134073 discloses an ink for inkjet recording having a good storage stability which can easily provide a dispersion excellent in ejectability in a bubble jet recording apparatus.
However, an ink for inkjet recording containing carbon black with a dispersant is technically disadvantageous in that it cannot be ejected stably and cannot provide a sufficient printing density. In order to solve these problems, JP-A No.8-3498 and JP-A No.10-120958 propose incorporating a functional group having dissociative hydrogen atoms in a predetermined amount or a salt thereof in carbon black to provide surface-modified carbon black, and thereby providing a dispersion of self-dispersible carbon black which can be self-dispersed without any surface active agent or polymer dispersant. Further, JP-A No.10-110127 discloses a method involving the incorporation of sulfone group in carbon black. Moreover, JP-A No.10-95941 proposes an ink for inkjet recording comprising the aforementioned surface-modified carbon black and glycolethes. The aforementioned self-dispersible carbon black provides a solution to the aforementioned problems but cannot achieve satisfactory results in high quality full-color printing because the water-soluble dyes to be combined therewith have a drastically deteriorated fastness.
In order to provide a full-color image having an excellent color reproducibility and fastness, the dyes constituting an image must satisfy the following requirements:    (1) All of the dyes have excellent absorption characteristics;    (2) A proper combination of dyes realizing a wide range of color reproduction;    (3) All of the dyes have a high fastness;    (4) No deterioration of fastness due to interaction of dyes occurs; and    (5) The dyes have well-balanced fastnesses.
However, since no reports have been made on the properties of dyes, that is, what structures or physical properties of the dyes can effectively act on their fastness, particularly with respect to oxidizing gases such as ozone, which is considered a problem in inkjet printing, no index for selection of dyes can be obtained. Further, it is extremely difficult to select dyes that are fast with respect to light as well.